1. Field of the Invention
The present invention relates, in general, to a method for fabricating a membrane and electrode assembly (hereinafter referred to as MEA ) and, more particularly, to use of a perfluorosulfonyl fluoride copolymer sheet as an electrolyte support which is so melt-processable that electrocatalysts are partially embedded thereinto upon hot pressing, thus providing effective three-phase boundaries in the interfaces between membrane and electrodes for polymer electrolyte membrane fuel cell(PEMFC).
2. Description of the Prior Art
Fuel cells are of clean technology, which convert chemical energy directly into electric energy, with electric power being produced as a part of a chemical reaction between the electrolyte and a fuel such as kerosine or industrial fuel gas, in which the hydrogen of carbohydrates such as methanol and natural gas and the oxygen of the air are usually used as a fuel and an oxidizer, respectively. In the 1970s, a fuel cell was developed as an electric power source for spacecrafts in the U.S. and since then, many studies have been made on application of fuel cells for general uses and great advances were achieved, particularly, in the U.S. and Japan.
Depending on the kinds of the electrolytes used, fuel cells are divided largely into alkali type(AFC), phosphate type(PAFC), melt carbonate type(MCFC), solid oxide type(SOFC) and polymer electrolyte type(PEMFC). Of them, the PEMFC are now much more actively developed as an electric power source, such as a power source for automobiles, a transportable electric source, an on-site power source, than those of other types in advanced countries including the U.S., Japan and Europe because of their superiority in many aspects. For instance, since the PEMFC utilize solid polymers as electrolytes, there occurs no erosion and evaporation of the electrolytes in the fuel cells. Further, the PEMFC can produce high current density per area, showing far superiority in output to other types as well as can be operated at low temperatures.
A PEMFC consists of a plurality of single cells, each comprising a proton-exchange membrane as an electrolyte to either side of which a positive and a negative electrode are respectively adhered by hot pressing and thus, a stack of the single cells can give an electricity-generating system ranging, in power, from kilowatts up to megawatts.
Such a single cell produces electricity according to the following mechanism. On the surface of a fuel electrode, the hydrogen atoms in the fuel react to donate electrons, becoming protons. The electrons flow to an oxygen electrode via an external circuit, producing electricity, whereas the protons move through the polymer electrolyte membrane(PEM) toward the oxygen electrode. At the oxygen electrode, the electrons reached reduce the oxygen molecules which, then, react with the protons to give water.
The performance of the PEMFC is dependent greatly on the electrodes. Each of the electrodes consists of an electrode support and an electrocatalyst layer. Usually, the electrode support is formed of a carbon cloth and the electrocatalyst layer is made porous by binding platinum-coated carbon powders with a water-proof binder (Pt/C).
A smaller contact area between electrodes and a catalyst is available for the PEMFC using solid electrolytes than for other fuel cells using liquid electrolytes. A typical PEMFC is, therefore, relatively low (&gt;10-20%) in catalyst utilization of Pt catalyst, showing a problem of requiring a high load of catalyst (4 mg/cm.sup.2).
As for the electrode reaction in a PEMFC, it occurs at a three-phase (electrolyte-catalyst-fuel gas phase) boundary, so that its rate changes greatly with the property of the boundary. For instance, the electrode reaction rate can be accelerated by bringing the electrodes into good contact with the electrolyte under a condition of uniformly diffusing the fuel gas in the electrodes. Accordingly, active research and development efforts have been and continue to be directed to the enhancement of the three-phase boundary, with the aim of increasing catalyst utilization and decreasing catalyst load.
There are known several conventional techniques for preparing effective three-phase boundaries. For example, ionomer-brushed or ionomer-impregnated electrodes, which are respectively obtained by coating a Nafion solution on the surface of a catalyst layer with the aid of a brush and drying it in an inert atmosphere or by forming a catalyst layer with a slurry combined with a Nafion solution, are adhered to both sides of a membrane by hot pressing to give an MEA, as disclosed by E. A. Ticianelli, C. R. Derouin, A. Redondo and S. Sirinivasan in J. Of Electrochemical. Soc., Vol. 135 No. 9, pp. 2209-2214, 1988 and by Aoyama, N. Eda and A. Ohta in J. Electrochem. Soc., Vol. 142 No. 2, pp. 463-468, 1995. The Nafion solution, which is prepared by mixing a proper amount of Nafion powder in a combination of water and alcohol, makes the contact between the electrode catalyst layer and the polymer electrolyte membrane(PEM) improved, giving rise to an increase in the utilization of the catalyst, platinum.
In the case of using such a Nafion solution to enhance the three-phase boundary, the electrodes are adhered to the polymer membrane by hot pressing, not in a fused state, but in a simple physically close contact state. Thus, the polymer membrane is simply brought into simple contact with the electrodes in plane, which has a limit in improving the utilization of the platinum catalyst. Additionally, a Nafion solution itself is expensive, increasing the cost of the electrodes.
Another technique for preparing effective three-phase boundaries is reported by E. J. Taylor, E. B. Anderson and N. R. K. Vilambi, in J. Electrochem. Soc., Vol. 139, No. 5, pp. L45-L46, 1992, which discloses that a Pt catalyst is electrodeposited locally in a contact site between an electrode and a polymer electrolyte by an electrochemical method using a platinum cation-containing electrolyte, such as Pt (NH.sub.3).sub.4 Cl.sub.2.
This electrochemical Pt catalyst-localization method is somewhat irksome in carrying out the electrodeposition and found to have a high possibility of delaminating an electrolyte membrane from an electrode layer.